Why doesn't Xenon fill it's 4th shell to 32 electons like it 'should' do?? 

Looking at this list of electons/shells: the rules seem a bit unclear...
http://en.wikipedia.org/wiki/Atomic_electron_configuration_table

It seems that up until element no. 56 (Barium) NOTHING can fill the
fourth shell higher than 18... but after 71 (Lutetium) they all fill
the fourth shell to 32...

As for elements 57-69, it's unclear...
As for the fifth shell, the pattern is even more unclear... 

Help! 

Can anyone provide a clear visual pattern as to max. number of
electons in outer shell, to explain these irregularities/patterns -
and WHY some elements don't bother filling up to max before using the
next shell up?

Thanks,
Kate

Electrons seek the position of lowest energy and try to get as close
to the nucleus as possible while staying out of the way of the other
electrons. Once you pass a certain point, the s and p shells of higher
orbitals are in a position of lower overall energy than the d and f
shells. This introduces some complexity into the order of filling and
is responsible for the properties of the transition elements.

http://chemlab.pc.maricopa.edu/periodic/filling.html

"Both definitions have their uses and supporters. The first has the
attraction of apparent simplicity and is the traditional usage.
However, many interesting properties of the transition elements as a
group are the result of their ability to contribute valence electrons
from s orbitals before d orbitals, a property which all members of the
d-block except zinc and scandium share, so the more restricted
definition is in many contexts the more useful. The d orbitals are
contributed after the s orbitals because once the d orbital begins to
fill its electrons move closer to the nucleus, leaving the s electrons
as the outermost."




http://www.ucc.ie/ucc/depts/chem/dolchem/html/comp/transmet.html

Another way to answer "Why doesn't Xenon fill it's 4th shell to 32
electons like it 'should' do??" has to do with the overlap of
principal shells.  This overlap first occurs with the n=3 and n=4
principal shells where the 3d subshell orbitals become available for
occupation prior to the 4p subshell orbitals because they are lower in
energy.  Consider the electron configuration of Kr, i.e., [Ar]4s2 3d10
4p6.....notice here that the principal shells n=1 (only one s orbital
allowed), n=2 (only one s and three p ortitals allowed ), n=3 (only
one s, three p, and 5 d orbitals allowed) are each filled in this case
BUT the n=4 (only one s, three p, five d, and seven f orbitals
allowed) principal shell is not filled as there are still unoccupied
4d and 4f orbitals available but at higher energies than the ground
state configuration provides.  As n increases, this overlap becomes
increasing more significant and complex.  As for Xe, its e-
configuration is [Kr]5s2 4d10 5p6....where again the principal shells
n=1, n=2, n=3 are each completely filled but now there are two
principal shells, n=4 and n=5, that are not 'completely' filled owing
to the existence of unoccupied 4f, 5d, 5f, and 5g orbitals available.
In effect, the use of the term 'should' has no relevance here as there
are NO quantum theory postulates that require electron configurations
of noble gases to have completely filled principal levels for their
highest n occupied shells or for their next lowest n occupied shells
and so forth.

Continuing with the question statement, "It seems that up until
element no. 56 (Barium) NOTHING can fill the
fourth shell higher than 18..."

As you move from Ba to the next elements in line in terms of atomic
number (consider an expanded periodic table for this thought),
speaking in terms of ground states, having just finished filling the
6s subshell (not the n=6 principal shell!), the 5d and/or 4f subshells
become the next available groups of orbitals to occupy.....both of
which are rather closely spaced in terms of energy....'for the most
part' completely filling into the 4f orbital (7 orbitals and 14
electrons allowed) prior to completely filling into the 5d orbital (5
orbitals and 10 electrons allowed).

Continuing on with question statement "but after 71 (Lutetium) they all fill
the fourth shell to 32..."......quite true as electrons have now
completely filled the 4f states (for that matter the n=4 principal
shell has now been filled as an f subshell has l=3 and there are no
more available l states to occupy in the n=4 principal shell) and
proceed to fill the next available states, i.e., the 5d states, 6p
states, 7s states, and so forth.

Continuing on with question statement "As for elements 57-69, it's
unclear...".......again quite true and not an easy thing for entrants
into the field of quantum mechanics to comprehend.  However, the
oddities of how the order of filling the 5d and 4f states are related
to similar arguments presented for exceptional cases to the aufbau
principal for earlier elements such as Cr, Cu, Mo, and Ag....the main
argument being that the resulting configurational ground state is
lower in energy than some state arbitrarily derived by simply using a
periodic table for guidance.

In many of the exceptional cases, extensive experimental evidence
(oftentimes of the spectroscopy type) to argue for the descriptions
that are now typically used for the elements' ground state electronic
configurations.  Being that reproducible experimental evidence can be
quite formidable to argue against, theorists had to argue what
alternative ground states would be reasonable to expect that would be
agreeable with the experimental findings.  In effect, these groups of
individuals had to apply the scientific method and apply modifications
to what had been developed.  Unfortunately some of these results
turned out to be not so easy to comprehend or accept outright upon an
introduction to the topics involved.  A further study into the matter
will not only shed some additional light on such confusion but also
many additional questions.
 
Continuing on with the question statement....."As for the fifth shell,
the pattern is even more unclear... " yes, and a few milleniums from
now, similar questions will undoubtedly be posed about the 6th, 7th,
8th, etc. principal shells.

ok so you cut and paste from a website a whole bunch of superfluous
nonsense,  basically repeating what i had clearly explained.

good job!

cyrilgrey.....your wise-crack is the 'superfluous' and unwarranted
component here!  I simply used the past 30+ of experience in chemistry
education to provide a little clearer input to a question coming from
someone who is obviously a beginning learner in chemistry.

This was the first comment that I've added to something like this and,
in fact, there was absolutely no cutting-and-pasting done whatsoever
in the effort.  Research this if you care to even attempt to back up
your false and self-centered accusation of plagerism.  I simply
recognized your input as being nonthorough and confusing at best,
especially to a beginning learner.

Sorry, but it sounds like you've had problems growing atop your totem
pole of reality.  More than likely, you're an uncomfortable type
that's never completely finished with removing the splinters of
failure we all deal with from time to time. May I suggest considering
a more effective "tweezing" mechanism.